The present invention relates to a color selective Si detector array with individually producible color filters of porous silicon. The invention also relates to a process for making same.
Porous silicon is produced very simply and inexpensively by anodic etching of monocrystalline silicon in a hydrofluoric acid solution. The porosity and microstructure of the porous silicon and thus also the optical refractive index depends, among other things, upon the doping of the starting material and the etching current density. By a change with time of the etching current density or by the use of a material with an appropriate doping profile, it is possible to produce porosity supergrids which can function as optical filters, for example, as Bragg Reflectors or Fabry-Perot Filters (M. G. Berger et al., J. Phys. D: Appl. Phys. 27, 1333, (1994)).
The application of such filters to an optical detector, for example, a pn-diode, changes the spectral sensitivity of the detector. The use of a number of detectors, with different filters enables, for example, color recognition or the detection of an energy spectrum of incident light with spatial resolution.
The object of the invention is to provide a detector array and a process for making same in which each individual detector is provided with an individual filter.
This object is achieved by an array in accordance with the invention which comprises a transistor, especially a pnp transistor, whose uppermost layer (collector layer) is transformed by anodic etching at least partly to porous silicon. The object is further attained by a process for producing a component or a multiplicity of components with optional layer sequences having whereby the layer sequence has transistor functions, for example, npn transistor function, heterobipolar transistor function or pnp transistor function with a low-doped intermediate layer for producing an improved photodetector effect.
According to this aspect of the invention, the upper layer or layer sequence is made porous, an electrolyte provided above the layer sequence serves as one of the transistor connections, and the other transistor connections are suitably biased (especially both of them) so that each individual transistor is supplied with the desired etching current for forming a desired filter. Further preferred or advantageous embodiments or variants include a transistor wherein the porous layer is formed by doping modulation in the upper p layer and/or by modulation of the etching current so that an optical filter. effect is achieved.
An array, especially a detector array, can be formed with a multiplicity such transistors. By respective electrical energization of the respective transistor electrodes and thus the formation of different etching currents, a porous silicon layer is formed during the etching of the respective transistor with optical characteristics individual to that transistor. The emitters of the individual transistors can be connected columnwise and the base terminals can be connected row-wise or vice versa so that with the aid of suitable row and column voltages, individual transistors or transistor groups are controllable.
The nonanodically etched p layer and/or n layer can be so formed as a pn photodetector that the porous layer is effective as an optical filter. The nonanodically etched p layer or layers of individual components can be electrically so connected row-wise and the n layer or n layers can be so electrically connected columnwise that by an optical excitation generated electrical signal can be tapped from individual pn diodes between the associated row conductors and column conductor.
The components according to the invention can be formed as a plurality of filters, especially three, with the colors red, blue and green as a color filter, especially for a color camera.
One or more components can form a reference for other color filter elements or for evaluating bright/dark information of incident light by etching.
A component or multiplicity of components according to the invention can have a layer sequence of the following pattern:
substrate-n-p-n-p, whereby the upper pnp transistor is used for the control of the anodic etching and the lower npn transistor is used as a phototransistor.
In the process of the invention in which the collector layer of a pnp transistor is converted by anodic etching at least partly into porous silicon, the etching current is controlled by the transistor functions by application of suitable biases to the electrolyte and the electrodes (base, emitter) which are not in direct contact with the electrolyte.
The invention provides a multiplicity of pnp transistors on an insulating or undoped or n-doped substrate. The layer which lies closest to the substrate serves as emitter, the n-layer serves as the base, and the p-layer lying at the surface serves as the collector. The individual transistors are electrically separated from one another by an appropriate insulation (for example, mesa etching, insulation-implantation or the like).
Advantageously, an array of cells can be fabricated in which each cell contains pixel detectors for different colors, for example, red, green, blue. In addition, the array can be controlled for column-wise and row-wise readout.